The present invention relates to a rare earth-based permanent magnet containing a rare earth element, iron and boron as the essential constituents and a method for the preparation thereof.
As is well known, rare earth-based magnetically anisotropic permanent magnets of R-T-B type, in which R is a rare earth element and T is iron or a combination of iron and cobalt, are now widely employed in the fields of electric and electronic instruments by virtue of their excellent magnetic properties and inexpensiveness for their high magnetic performance.
While the magnetically anisotropic rare earth-based permanent magnets of the above mentioned type are usually prepared by the sintering method in which particles of the magnetic alloy are compression-molded into a powder compact in a magnetic field to align the alloy particles relative to the easy magnetization axes of the particles and the powder compact is subjected to a heat treatment to effect sintering, the rare earth-based alloy powder is prepared by the melt-pulverization method or by the direct reduction-diffusion method. In the former method, the constituent elements each in the metallic or elementary form taken in a specified proportion are melted together into an alloy melt which is cast into an ingot to be pulverized. In the latter method, the starting materials including an oxide of the rare earth element, powder of iron and powder of a ferroboron taken in a specified proportion are thoroughly blended with further admixture of a powder of metallic calcium as the reducing agent and the powder blend is subjected to a heat treatment to effect the reducing reaction of the rare earth oxide with calcium and simultaneous diffusion thereof with the particles of iron and/or ferroboron.
Though advantageous in respect of the easy controllability of the alloy composition obtained by the method, the melt-pulverization method has disadvantages that mefting of the constituents must be carried out at a high temperature under a strictly controlled inert gas atmosphere and the rare earth metal as one of the starting materials is relatively expensive. Further, the alloy particles obtained by this method have a problem that the alloy has a metallographic structure containing an incipient crystalline phase of iron precipitated in the course of casting along with segregation of a phase rich in the content of the rare earth element.
In contrast thereto, the direct reduction-diffusion method is advantageous because the rare earth oxide as one of the starting materials is relatively inexpensive as compared with the rare earth metal, the reaction temperature can be relatively low and the alloy as the reaction product is obtained in the form of a powder so that the process of crushing into a coarse powder can be omitted. On the other hand, this method has some problems that the alloy composition can be controlled only with difficulties and the oxygen content of the alloy is liable to be increased by the washing treatment of the reaction product with water undertaken in order to remove the unreacted calcium metal or calcium oxide formed as a reaction product of the reducing reaction. Further, as an inherence of the method for the formation of an intermetallic compound, each of the particles of the host phase R.sub.2 Fe.sub.14 B is surrounded by a layer of an auxiliary phase rich in the content of the rare earth element which is more susceptible to oxidation than the host phase to cause uncontrollable variation of the magnetic properties of the magnets prepared from the alloy powder though somewhat advantageous relative to the problem of segregation as compared with the melt-pulverization method.
It is generally understood that the magnetic properties of a permanent magnet of the R-Fe-B type can be improved by increasing the fraction of the magnetically hard host phase of R.sub.2 Fe.sub.14 B. In the melt-pulverization method, however, an alloy composition approximating the composition of the host phase R.sub.2 Fe.sub.14 B has another problem that segregation of the coarse incipient crystalline phase of iron and the rare earth-rich phase is increased along with an increased difficulty in the pulverization of the alloy ingot.
As a solution of the above mentioned problems, the two-alloy method is proposed, in which a powder blend is prepared from a principal alloy from which the ferromagnetic host phase of R.sub.2 Fe.sub.14 B is formed and an auxiliary alloy rich in the content of the rare earth element, which serves to promote sintering of the powder compact and exhibits a cleaning effect on the surface of the host phase particles, and the powder blend is further pulverized and subjected to a sintering treatment in a conventional manner. In this two-alloy method, it is important to undertake homogenization of the alloy by a heat treatment at an appropriate temperature in order to reduce segregation of coarse incipient crystals of iron in the alloy for the formation of the host phase.
With an object to prevent growth of the crystal grains and precipitation and growth of the incipient iron crystals as the defects of the melt-pulverization method, the so-called strip-casting method has been developed in which a melt of the alloy is ejected onto the surface of a rotating roller consisting of a single roller or twin rollers of copper so as to give a thin alloy ribbon formed by quenching of the melt. In this strip-casting method, various factors affecting the rate of solidification of the alloy melt including revolution of the quenching roller, rate of melt ejection and atmosphere inside the cooling chamber can be controlled so as to prevent occurrence of coarse incipient crystals of iron and to accomplish a thin alloy ribbon of the uniform host phase of R.sub.2 Fe.sub.14 B having an adequate particle diameter.
As a method for the preparation of a magnetically isotropic rare earth-based permanent magnet, the so-called melt-spun method is proposed, in which, similarly to the above described strip-casting method, a melt of the alloy is ejected onto the surface of a quenching roller consisting of a single roller or twin rollers to give a thin alloy ribbon. Different from the strip-casting method, the rate of solidification of the alloy melt in the melt-spun method is much greater than in the strip-casting method so that the thin alloy ribbon obtained by this method has an amorphous or microcrystalline structure. An isotropic permanent magnet is obtained by subjecting the alloy in the form of a thin alloy ribbon to a heat treatment under appropriate conditions to effect crystal growth of the R.sub.2 Fe.sub.14 B phase as the host phase exhibiting a coercive force.
This method is applied in recent years to the preparation of a composite magnet material consisting of a combination of a magnetically hard phase of R.sub.2 Fe.sub.14 B and a magnetically soft phase of Fe or Fe.sub.3 B. In these composite magnetic materials, the magnetically hard and soft phases are dispersed each in the other in a fineness of nanometer order dimensions entering magnetic exchange coupling so that the demagnetization curve of the magnet resembles that obtained with a single magnetically hard phase. The magnet of this type is sometimes called an exchange spring magnet since the magnetization of the magnet along the demagnetization curve on the hysteresis loop exhibits a unique and unordinary behavior of irreversible spring-back as the external magnetic field is decreasing.
As a result of the above described various improvements, the rare earth-based magnetically anisotropic sintered permanent magnets, now expected to be under mass production in the near future, have been upgraded so as to have a maximum energy product (BH).sub.max of as large as 50 MGOe approaching the theoretical upper limit of 64 MGOe. Since the principal phase of those magnets is R.sub.2 Fe.sub.14 B having a low saturation magnetization as compared with iron per se and the like, however, it is generally understood that improvements in the magnetic properties of the magnets of this type will shortly get at a limiting bar hardly surpassed in practice.
Since the exchange-spring magnet described above contains, besides the host phase of R.sub.2 Fe.sub.14 B, phases having a higher saturation magnetization such as Fe and Fe.sub.3 B, on the other hand, the magnet has a potentiality of exhibiting superior magnetic properties but the phase of R.sub.2 Fe.sub.14 B, which is formed by a heat treatment of the alloy obtained by the melt-spun method, is isotropic with randomly oriented easy magnetization axes of the magnetic particles. This is the reason for the failure of obtaining a high-performance, magnetically anisotropic permanent magnet like a sintered magnet.